Broad-bandwidth optical fiber

ABSTRACT

The present invention embraces an optical fiber that includes a central core having an alpha refractive index profile with respect to an outer optical cladding. The optical fiber also includes an inner cladding and a buried trench. The central core includes a core matrix doped with at least fluorine and a dopant element that increases refractive index. The optical fiber typically has reduced bending losses and cladding effect as well as a high bandwidth at the wavelengths of 850 nanometers and 1300 nanometers for high-data-rate applications.

CROSS-REFERENCE TO PRIORITY APPLICATIONS

This application hereby claims the benefit of commonly assigned, pending French Application No. 1150734 for a “Fibre Optique A Large Bande Passante Et A Faibles Pertes Par Courbure” (filed Jan. 31, 2011 at the National Institute of Industrial Property (France)), which is hereby incorporated by reference in its entirety.

This application further claims the benefit of commonly assigned U.S. Provisional Patent Application Ser. No. 61/438,372 for a “Fibre Optique A Large Bande Passante Et A Faibles Pertes Par Courbure” (filed Feb. 1, 2011), which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of optical fibers, and, more specifically, to an optical fiber presenting reduced bending losses and a broad bandwidth for broadband applications.

BACKGROUND

An optical fiber (i.e., a glass fiber typically surrounded by one or more coating layers) conventionally includes an optical fiber core, which transmits and/or amplifies an optical signal, and an optical cladding, which confines the optical signal within the core. Accordingly, the refractive index of the core n_(c) is typically greater than the refractive index of the optical cladding n_(g) (i.e., n_(c)>n_(g)). The refractive index difference between the central core and the optical cladding is typically obtained by introducing dopants into the central core and/or the optical cladding.

Typically, the central core and the optical cladding of an optical fiber are obtained by vapor deposition, such as inside chemical vapor deposition (CVD), outside vapor deposition (OVD), vapor axial deposition (VAD), etc. In a typical inside CVD-type method, the deposition tube and an overcladding or sleeving may constitute the outer optical cladding. The central core is formed of a matrix that may include one or more doping elements. The central core matrix is typically made of silica.

For optical fibers, the refractive index profile is generally classified according to the graphical appearance of the function that associates the refractive index with the radius of the optical fiber. Conventionally, the distance r to the center of the optical fiber is shown on the x-axis, and the difference between the refractive index (at radius r) and the refractive index of the optical fiber's outer cladding (e.g., an outer optical cladding) is shown on the y-axis. The refractive index profile is referred to as a “step” profile, “trapezoidal” profile, “triangular” profile, or “parabolic” profile (e.g., a graded profile or an “alpha” profile) for graphs having the respective shapes of a step, a trapezoid, a triangle, or a parabola. These curves are generally representative of the optical fiber's theoretical or set profile. Constraints in the manufacture of the optical fiber, however, may result in a slightly different actual profile.

Generally speaking, two main categories of optical fibers exist: multimode fibers and single-mode fibers. In a multimode optical fiber, for a given wavelength, several optical modes are propagated simultaneously along the optical fiber. In a single-mode optical fiber, the signal propagates in a fundamental LP01 mode that is guided in the fiber core, while the higher order modes (e.g., the LP11 mode) are strongly attenuated.

The typical diameter of a single-mode or multimode glass fiber is 125 microns. The core of a multimode optical fiber typically has a diameter of between about 50 microns and 62.5 microns, whereas the core of a single-mode optical fiber typically has a diameter of between about 6 microns and 9 microns. Multimode systems are generally less expensive than single-mode systems, because multimode light sources, connectors, and maintenance can be obtained at a lower cost.

Multimode optical fibers are commonly used for short-distance applications requiring a broad bandwidth, such as local networks or LAN (local area network). Multimode optical fibers have been the subject of international standardization under the ITU-T G.651.1 recommendations, which, in particular, define criteria (e.g., bandwidth, numerical aperture, and core diameter) that relate to the requirements for optical fiber compatibility. The ITU-T G.651.1 standard (July 2007) is hereby incorporated by reference in its entirety.

In addition, the OM3 standard has been adopted to meet the demands of high-bandwidth applications (i.e., a data rate higher than 1 GbE) over long distances (i.e., distances greater than 300 meters). The OM3 standard is hereby incorporated by reference in its entirety. With the development of high-bandwidth applications, the average core diameter for multimode optical fibers has been reduced from 62.5 microns to 50 microns.

Typically, an optical fiber should have the broadest possible bandwidth to perform well in a high-bandwidth application. For a given wavelength, the bandwidth of an optical fiber may be characterized in several different ways. Typically, a distinction is made between the so-called “overfilled launch” condition (OFL) bandwidth and the so-called “effective modal bandwidth” condition (EMB). The acquisition of the OFL bandwidth assumes the use of a light source exhibiting uniform excitation over the entire radial surface of the optical fiber (e.g., using a laser diode or light emitting diode (LED)).

Recently developed light sources used in high-bandwidth applications, such as VCSELs (Vertical Cavity Surface Emitting Lasers), exhibit an inhomogeneous excitation over the radial surface of the optical fiber. For this kind of light source, the OFL bandwidth is a less suitable measurement, and so it is preferable to use the effective modal bandwidth (EMB). The calculated effective bandwidth (EMBc) estimates the minimum EMB of a multimode optical fiber independent of the kind of VCSEL used. The EMBc is obtained from a differential-mode-delay (DMD) measurement (e.g., as set forth in the FOTP-220 standard).

An exemplary method of measuring DMD and calculating the effective modal bandwidth can be found in the FOTP-220 standard, which is hereby incorporated by reference in its entirety. Further details on this technique are set forth in the following publications, each of which is hereby incorporated by reference: P. F. Kolesar and D. J. Mazzarese, “Understanding Multimode Bandwidth and Differential Mode Delay Measurements and Their Applications,” Proceedings of the 51st Int'l Wire and Cable Symposium, 2002, pp. 453-460; and Doug Coleman and Phillip Bell, “Calculated EMB Enhances 10 GbE Performance Reliability for Laser-Optimized 50/125 μm Multimode Fiber,” Corning Cable Systems Whitepaper (March 2005).

FIG. 1 shows a schematic diagram of a prophetic DMD measurement according to the criteria of the FOTP-220 standard as published in its TIA SCFO-6.6 version of Nov. 22, 2002. FIG. 1 schematically represents a part of an optical fiber (i.e., an optical core surrounded by an outer cladding). A DMD graph is obtained by successively injecting into the multimode optical fiber 20 a light pulse 21 having a given wavelength λ₀ with a radial offset 24 between each successive pulse. The delay of each pulse is then measured after a given length of fiber L. Multiple identical light pulses (i.e., light pulses having the same amplitude, wavelength, and frequency) are injected with different radial offsets with respect to the center of the multimode optical fiber's core 22. In order to characterize an optical fiber with a 50-micron diameter, the FOTP-220 standard recommends that individual measurements be carried out at radial offset intervals of about two microns or less (e.g., twenty-six individual measurements). From these measurements a map 23 of the DMD, or “DMD graph,” may be generated depicting the pulse delay (e.g., in nanoseconds) as a function of radial offset (e.g., in microns). From the DMD graph 23, the modal dispersion and calculated effective modal bandwidth (EMBc) may be determined.

The TIA-492AAAC-A standard, which is hereby incorporated by reference in its entirety, specifies the performance requirements for 50-micron-diameter multimode optical fibers used over long distances in Ethernet high-bandwidth transmission network applications. The OM3 standard requires, at a wavelength of 850 nanometers, an EMB of at least 2,000 MHz·km. The OM3 standard assures error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 300 meters. The OM4 standard requires, at a wavelength of 850 nanometers, an EMB of at least 4,700 MHz·km to obtain error-free transmissions for a data rate of 10 Gb/s (10 GbE) up to a distance of 550 meters. The OM4 standard is hereby incorporated by reference in its entirety.

The OM3 and OM4 standards may also guarantee an OFL bandwidth at a wavelength of 1300 nanometers that is greater than 500 MHz·km. A broadband optical fiber that provides high-bandwidths at the wavelengths 850 nanometers and 1300 nanometers makes fast signal transmission possible over a broad band of wavelengths.

In a multimode optical fiber, the difference between the propagation times, or group delay times, of the several modes along the optical fiber determine the bandwidth of the optical fiber. In particular, for the same propagation medium (i.e., in a step-index multimode optical fiber), the different modes have different group delay times. This difference in group delay times results in a time lag between the pulses propagating along different radial offsets of the optical fiber.

For example, as shown in the DMD graph 23 on the right side of FIG. 1, a time lag is observed between the individual pulses. This DMD graph depicts each individual pulse in accordance with its radial offset in microns (y-axis) and the time in nanoseconds (x-axis) the pulse took to pass along a given length of the optical fiber.

As depicted in FIG. 1, the location of the peaks along the x-axis varies, which indicates a time lag (i.e., a delay) between the individual pulses. This delay causes a broadening of the resulting light pulse. Broadening of the light pulse increases the risk of the pulse being superimposed onto a trailing pulse and reduces the bandwidth (i.e., data rate) supported by the optical fiber. The bandwidth, therefore, is directly linked to the group delay time of the optical modes propagating in the multimode core of the optical fiber. Thus, to guarantee a broad bandwidth, it is desirable for the group delay times of all the modes at a given wavelength to be identical. Stated differently, the intermodal dispersion should be zero, or at least minimized, for a given wavelength.

To reduce intermodal dispersion, the multimode optical fibers used in telecommunications generally have a core with a refractive index that decreases progressively from the center of the optical fiber to its interface with a cladding (i.e., an “alpha” core profile). Such an optical fiber has been used for a number of years, and its characteristics have been described in “Multimode Theory of Graded-Core Fibers” by D. Gloge et al., Bell system Technical Journal 1973, pp. 1563-1578, and summarized in “Comprehensive Theory of Dispersion in Graded-Index Optical Fibers” by G. Yabre, Journal of Lightwave Technology, February 2000, Vol. 18, No. 2, pp. 166-177. Each of the above-referenced articles is hereby incorporated by reference in its entirety.

A graded-index profile (i.e., an alpha-index profile) can be described by a relationship between the refractive index value n and the distance r from the center of the optical fiber according to the following equation:

$n = {n_{1}\sqrt{1 - {2\;{\Delta\left( \frac{r}{a} \right)}^{\alpha}}}}$

wherein,

α≧1, and α is a non-dimensional parameter that is indicative of the shape of the index profile;

n₁ is the maximum refractive index of the optical fiber's core;

a is the radius of the optical fiber's core; and

$\Delta = \frac{\left( {n_{1}^{2} - n_{0}^{2}} \right)}{2n_{1}^{2}}$

where n₀ is the minimum refractive index of the multimode core, which may correspond to the refractive index of the outer cladding (most often made of silica).

A multimode optical fiber with a graded index (i.e., an alpha profile) therefore has a core profile with a rotational symmetry such that along any radial direction of the optical fiber the value of the refractive index decreases continuously from the center of the optical fiber's core to its periphery. When a multimode light signal propagates in such a graded-index core, the different optical modes experience differing propagation mediums (i.e., because of the varying refractive indices). This, in turn, affects the propagation speed of each optical mode differently. Thus, by adjusting the value of the parameter α, it is possible to obtain a group delay time that is virtually equal for all of the modes. Stated differently, the refractive index profile can be modified to reduce or even eliminate intermodal dispersion.

In practice, however, a manufactured multimode optical fiber has a graded-index central core surrounded by an outer cladding of constant refractive index. The core-cladding interface interrupts the core's alpha-index profile. Consequently, the multimode optical fiber's core never corresponds to a theoretically perfect alpha profile (i.e., the alpha set profile). The outer cladding accelerates the higher-order modes with respect to the lower-order modes. This phenomenon is known as the “cladding effect.” In DMD measurements, the responses acquired for the highest radial positions (i.e., nearest the outer cladding) exhibit multiple pulses, which results in a temporal spreading of the response signal. Therefore, bandwidth is diminished by this cladding effect.

Multimode optical fibers are commonly used for short-distance applications requiring a high bandwidth, such as local area networks (LANs). In such applications, the optical fibers may be subjected to accidental or otherwise unintended bending, which can give rise to signal attenuation and modify the mode power distribution and the bandwidth of the optical fiber.

It is therefore desirable to achieve multimode optical fibers that are unaffected by bends having a radius of curvature of less than 10 millimeters. One proposed solution involves adding a depressed trench between the core and the cladding. Nevertheless, the position and the depth of the trench can significantly affect the optical fiber's bandwidth.

European Patent No. 1498753, which is hereby incorporated by reference in its entirety, discloses an optical fiber having a bandwidth that is greater than 2000 MHz·km at wavelengths of 850 nanometers and 1300 nanometers. Nevertheless, the optical fiber according to European Patent No. 1498753 does not ensure compliance with the OM4 standard nor does it take account of the trench effect that minimizes bending losses.

European Patent No. 1503230 and its counterpart U.S. Pat. Nos. 7,421,172, and 7,315,677, each of which is hereby incorporated by reference, disclose optical fibers in which the central core refractive index difference is obtained by co-doping with germanium and fluorine. The fluorine and germanium concentrations in the central core are adjusted to optimize the bandwidth at wavelengths of 850 nanometers and 1300 nanometers. Nevertheless, the optical fibers according to European Patent No. 1503230 and U.S Pat. No. 7,315,677 neither comply with the OM4 standard at a wavelength of 850 nanometers nor take account of the effect of a trench that minimizes bending losses.

International Publication No. WO 2009/054715 and its counterpart U.S. Pat. No. 8,009,950, each of which is hereby incorporated by reference, disclose an optical fiber in which the central core refractive index difference is obtained by co-doping with germanium and fluorine. The optical fiber according to International Publication No. WO 2009/054715 presents a buried trench at the central core's periphery. The graded-index central core extends to a refractive index below the refractive index of the outer optical cladding. Such an extension of the central core can give rise to an increase in the size of the central core, resulting in an optical fiber that is incompatible with the OM3 and OM4 standards. Extending the central core can also give rise to losses due to the propagation of leakage modes intrinsic to buried-trench geometry.

Therefore, a need exists for a graded-index multimode optical fiber having reduced bending losses and cladding effect as well as a high bandwidth at the wavelengths of 850 nanometers and 1300 nanometers for high-data-rate applications.

SUMMARY

Accordingly, in one aspect, the present invention embraces an optical fiber that includes a central core surrounded by an outer optical cladding. The central core has an outer radius r₁ and an alpha-index profile with respect to the outer optical cladding. The central core includes a core matrix doped with at least fluorine and a dopant element that increases refractive index. An inner cladding is positioned between the central core and the outer optical cladding (e.g., immediately surrounding the central core). The inner cladding has (i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive index difference Δn₂ with respect to the outer optical cladding. A buried trench is positioned between the inner cladding and the outer optical cladding (e.g., immediately surrounding the inner cladding). The buried trench has (i) an outer radius r₃, (ii) a width w₃, and (iii) a refractive index difference Δn₃ with respect to the outer optical cladding. Within the optical fiber's central core, the atomic ratio C_(F) of fluorine to all of the constituents of the doped core matrix typically increases from the center of the optical fiber (e.g., the center of the central core) to the central core's outer radius r₁. At the central core's outer radius r₁, the atomic ratio C_(F) of fluorine typically has a maximum value C_(F,MAX) of between about 8.5×10⁻³ and 57×10⁻³.

In an exemplary embodiment, the buried trench's width w₃ is between about 2 microns and 10 microns.

In another exemplary embodiment, the buried trench's refractive index difference Δn₃ is between about −15×10⁻³ and −6×10⁻³.

In yet another exemplary embodiment, the dopant element that increases the central core's refractive index is germanium.

In yet another exemplary embodiment, the core matrix is silica.

In yet another exemplary embodiment, at the central core's outer radius r₁, the buried trench's refractive index difference Δn₃ and the atomic ratio of fluorine's maximum value C_(F,MAX) satisfy the following inequality: C _(F,MAX)×1000>−8.97×1000×Δn ₃−61.54.

In yet another exemplary embodiment, at the central core's outer radius r₁, the buried trench's refractive index difference Δn₃ and the atomic ratio of fluorine's maximum value C_(F,MAX) satisfy the following inequality: C _(F,MAX)×1000>−8.28×1000×Δn ₃−48.28.

In yet another exemplary embodiment, at the central core's outer radius r₁, the buried trench's refractive index difference Δn₃ and the atomic ratio of fluorine's maximum value C_(F,MAX) satisfy the following inequality: C _(F,MAX)×1000>−8×1000×Δn ₃−42.28.

In yet another exemplary embodiment, within the optical fiber's central core, the atomic ratio C_(F) of fluorine to all of the constituents of the doped core matrix is a function C_(F)(r) of the radial distance r from the center of the optical fiber and C_(F)(r) is defined by the following equation:

${C_{F}(r)} = {C_{F}^{(1)} + {\left( \frac{r}{r_{1}} \right)^{p} \cdot \left( {C_{F,{MAX}} - C_{F}^{(1)}} \right)}}$

where:

p is a constant substantially equal to 2; and

C_(F) ⁽¹⁾ is a positive constant less than 1.25×10⁻³.

In yet another exemplary embodiment, the alpha-index profile of the central core has an alpha parameter α of between about 1.9 and 2.1 (e.g., between about 2.04 and 2.1, such as between about 2.05 and 2.08).

In yet another exemplary embodiment, the refractive index difference between the central core and the outer cladding has a maximum value Δn₁ of between about 10×10⁻³ and 18×10⁻³.

In yet another exemplary embodiment, the refractive index difference between the central core and the outer cladding has a maximum value Δn₁ of between about 11×10⁻³ and 16×10⁻³.

In yet another exemplary embodiment, the inner cladding's width w₂ is between about 0.5 micron and 2 microns.

In yet another exemplary embodiment, the inner cladding's refractive index difference Δn₂ is between about −0.2×10⁻³ and 2×10⁻³.

In yet another exemplary embodiment, the buried trench's width w₃ is between about 3 microns and 5 microns.

In yet another exemplary embodiment, the multimode optical fiber has a numerical aperture of 0.2±0.015 (i.e., between 0.185 and 0.215).

In yet another exemplary embodiment, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.2 dB (e.g., less than about 0.1 dB).

In yet another exemplary embodiment, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, the multimode optical fiber has bending losses of less than about 0.05 dB (e.g., less than about 0.01 dB).

In yet another exemplary embodiment, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 1300 nanometers, the multimode optical fiber has bending losses of less than about 0.5 dB (e.g., less than about 0.3 dB).

In yet another exemplary embodiment, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 1300 nanometers, the multimode optical fiber has bending losses of less than about 0.2 dB (e.g., less than about 0.1 dB).

In yet another exemplary embodiment, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 1300 nanometers, the multimode optical fiber has bending losses of less than about 0.05 dB.

In yet another exemplary embodiment, at a wavelength of 850 nanometers, the multimode optical fiber has an OFL bandwidth of about 1500 MHz·km or greater.

In yet another exemplary embodiment, at a wavelength of 850 nanometers, the multimode optical fiber has an OFL bandwidth of about 3500 MHz·km or greater.

In yet another exemplary embodiment, at a wavelength of 1300 nanometers, the multimode optical fiber has an OFL bandwidth of about 500 MHz·km or greater.

In yet another exemplary embodiment, at a wavelength of 1300 nanometers, the multimode optical fiber has an OFL bandwidth of about 600 MHz·km or greater.

In yet another exemplary embodiment, at a wavelength of 850 nanometers, the multimode optical fiber has a dispersion mode delay DMDext value as acquired with a mask covering the outer portion of the central core in accordance with the FOTP-220 standard of about 0.33 ps/m or less.

In yet another exemplary embodiment, at a wavelength of 850 nanometers, the multimode optical fiber has a dispersion mode delay DMDext value as acquired with a mask covering the outer portion of the central core in accordance with the FOTP-220 standard of about 0.25 ps/m or less.

In yet another exemplary embodiment, at a wavelength of 850 nanometers, the multimode optical fiber has a dispersion mode delay DMDext value as acquired with a mask covering the outer portion of the central core in accordance with the FOTP-220 standard of about 0.14 ps/m or less.

In another aspect, the present invention embraces a multimode optical system that includes an optical fiber in accordance with the foregoing.

In an exemplary embodiment, the optical system presents a data rate of about 10 Gb/s or greater over about 100 meters or greater (e.g., over about 300 meters or greater).

The foregoing illustrative summary, as well as other exemplary objectives and/or advantages of the invention, and the manner in which the same are accomplished, are further explained within the following detailed description and its accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an exemplary DMD measurement method and graph.

FIG. 2 graphically depicts the refractive index profile of an exemplary optical fiber according to the present invention.

FIG. 3 graphically depicts the concentrations of fluorine and germanium as a function of radial position in a comparative optical fiber.

FIG. 4 graphically depicts the concentrations of fluorine and germanium as a function of radial position in another comparative optical fiber.

FIG. 5 graphically depicts the concentrations of fluorine and germanium as a function of radial position in an exemplary optical fiber according to the present invention.

FIG. 6 graphically depicts the relationship between (i) the atomic ratio of fluorine to all of the constituents of the doped core matrix and (ii) the refractive index difference attributable to the fluorine doping.

FIG. 7A graphically depicts, for different alpha parameter values and difference buried trench refractive index differences, the overfilled launch (OFL) bandwidth at a wavelength of 1300 nanometers as a function of the overfilled launch (OFL) bandwidth at a wavelength of 850 nanometers for comparative optical fibers.

FIG. 7B graphically depicts the overfilled launch (OFL) bandwidth at a wavelength of 1300 nanometers as a function of the overfilled launch (OFL) bandwidth at a wavelength of 850 nanometers for exemplary optical fibers.

FIG. 8 graphically depicts, for three respective values of the buried trench's refractive index difference, the overfilled launch (OFL) bandwidth at a wavelength of 1300 nanometers as a function of the value of the atomic ratio of fluorine to all of the constituents of the doped core matrix at the end of the central core's alpha-profile.

FIG. 9 graphically depicts the overfilled launch (OFL) bandwidth at a wavelength of 1300 nanometers as a function of (i) the value of the atomic ratio of fluorine to all of the constituents of the doped core matrix at the end of the central core's alpha-profile and (ii) the buried trench's refractive index difference.

FIG. 10 graphically depicts the desired alpha parameter for achieving a maximum bandwidth at a wavelength of 850 nanometers as a function of the value of the atomic ratio of fluorine to all of the constituents of the doped core matrix at the end of the central core's alpha-profile.

FIG. 11 graphically depicts the attenuation at a wavelength of 850 nanometers as a function of the value of the atomic ratio of fluorine to all of the constituents of the doped core matrix at the end of the central core's alpha-profile.

DETAILED DESCRIPTION

The present invention embraces a multimode optical fiber that achieves reduced bending losses and cladding effect as well as a high bandwidth at the wavelengths of 850 nanometers and 1300 nanometers for high-data-rate applications. The optical fiber includes a central core having an alpha-index profile with respect to an outer optical cladding that surrounds the central core. The central core includes a core matrix doped with at least fluorine and a dopant element that increases refractive index (e.g., fluorine or phosphorous). An inner cladding is typically positioned between the central core and the outer optical cladding (e.g., immediately surrounding the central core). A buried trench is positioned between the inner cladding and the outer optical cladding (e.g., immediately surrounding the inner cladding). For reasons of cost, the outer optical cladding is typically made of natural silica, but it may also be made of doped silica.

In exemplary optical fibers according to the present invention, the alpha-index profile of the central core, the inner cladding, and the buried trench serve to diminish bending losses while guaranteeing a broad bandwidth at a wavelength of 850 nanometers. The concentrations of fluorine and of the dopant element that increases the refractive index in the central core improve the bandwidth at a wavelength of 1300 nanometers.

FIG. 2 depicts the refractive index profile of an exemplary optical fiber in accordance with the present invention. The exemplary optical fiber is a multimode optical fiber that includes a central core surrounded by an outer optical cladding.

As depicted, the exemplary optical fiber also includes an inner cladding positioned between the central core and the outer optical cladding (e.g., immediately surrounding the central core). A buried trench is positioned between the inner cladding and the outer optical cladding (e.g., immediately surrounding the inner cladding).

The central core has an outer radius r₁ and an alpha-index profile with respect to the outer optical cladding. The central core's alpha-index profile has a maximum refractive index difference Δn₁ with respect to the outer optical cladding at the beginning of the alpha-profile. Typically, the “beginning of the alpha-profile” corresponds to the central core's maximum refractive index difference with respect to the outer optical cladding. Furthermore, the central core's maximum refractive index difference typically corresponds to the center of the central core.

At the end of the alpha profile, the central core has a minimum refractive index that corresponds to a refractive index difference Δn_(end) (not shown) with respect to the outer optical cladding. Typically, the “end of the alpha-profile” corresponds to the radial distance from the center of the central core at which the central core's refractive index profile is no longer an alpha-profile. Furthermore, the central core's outer radius r₁ typically corresponds to the distance between the beginning of the alpha-profile and the end of the alpha-profile. Thus, the central core's refractive index difference Δn_(end) typically occurs at the central core's outer radius r₁.

The central core's refractive index difference Δn_(end) is typically about zero (i.e., the central core's alpha-index profile may have a minimum refractive index value at the central core's outer radius r₁ that is equal to that of the outer optical cladding). That said, central core's refractive index difference Δn_(end) may be negative (i.e., the central core's alpha-index profile may have a minimum refractive index value at the central core's outer radius r₁ that is less than that of the outer cladding). Furthermore, the central core's refractive index difference Δn_(end) may be positive (i.e., the central core's alpha-index profile may have a minimum refractive index value at the central core's outer radius r₁ that is greater than that of the outer cladding).

In exemplary embodiments, the central core's outer radius r₁ is 25 microns±2 microns (i.e., between about 23 microns and 27 microns). The alpha-profile's maximum refractive index difference Δn₁ is typically between 10×10⁻³ and 18×10⁻³ (e.g., between 11×10⁻³ and 16×10⁻³). The central core's alpha-profile typically has an alpha parameter α of between 1.9 and 2.1 (e.g., between 2.04 and 2.1, such as between 2.05 and 2.08).

The central core includes a core matrix, defined as the main constituent of the central core, in which other constituents, such as dopants or dopant elements, may be included. Typically, in an optical fiber, the refractive index difference relative to the outer optical cladding is obtained by including dopants in the core matrix that modify the refractive index. For example, germanium (e.g., in the form of germanium dioxide GeO₂) increases the refractive index, whereas fluorine decreases the refractive index. The matrix of the central core (i.e., the core matrix) may be made of silica (i.e., silicon dioxide, SiO₂).

In this regard, the exemplary refractive index profile shown in FIG. 2 may be obtained with the fluorine and germanium concentration profiles shown in FIG. 3. The concentration profiles shown in FIG. 3 are prophetic comparative examples. In FIG. 3, the germanium concentration curve is represented by a continuous line and the fluorine concentration curve is represented by a dashed line. As noted, refractive index profiles, such as in FIG. 2, are generally representative of the optical fiber's theoretical or set profile.

As shown in FIG. 3, for radial positions less than the central core's outer radius r₁, the concentration of fluorine is zero and the concentration of germanium decreases with a gradient corresponding to the alpha-index profile of the central core. For radial positions between the central core's outer radius r₁ and the inner cladding's outer radius r₂, the fluorine concentration is zero and the germanium concentration is sufficient to achieve the inner cladding's refractive index Δn₂ relative to the outer optical cladding. For radial positions between the inner cladding's outer radius r₂ and the buried trench's outer radius r₃, the germanium concentration is zero and the fluorine concentration is sufficient to achieve the buried trench's refractive index difference Δn₃ relative to the outer optical cladding.

In the comparative example shown in FIG. 3, the refractive index differences are obtained either using germanium alone or using fluorine alone. The refractive index profile of FIG. 2 may also be obtained using co-doping of germanium and fluorine as shown in FIG. 4. The concentration profiles shown in FIG. 4 are prophetic comparative examples. In FIG. 4, the germanium concentration curve is represented by a continuous line and the fluorine concentration curve is represented by a dashed line.

As shown in FIG. 4, for radial positions less than the central core's outer radius r₁, the fluorine concentration increases progressively from the center of the optical fiber towards the end of the alpha profile, and the germanium concentration decreases progressively from the center of the optical fiber towards the end of the alpha profile. Thus, in the comparative example of FIG. 4, and contrary to the example of FIG. 3, the alpha index profile is obtained by combining germanium, which increases the refractive index, with fluorine, which decreases the refractive index.

The use of such co-doping provides better control over the resulting refractive index profile. The appearance of the refractive index profile is controlled more finely by using two dopants instead of only one. Furthermore, each dopant imparts losses to the optical signal being transmitted, thereby decreasing the bandwidth of the optical fiber. These losses are a function of the dopant, of the concentration of the dopant, and of the wavelength of the transmitted fiber signal.

A doping concentration profile of an optical fiber may be adapted to achieve a broad bandwidth at a given wavelength. Nevertheless, the same doping concentration profile may give rise to a narrow bandwidth at other wavelengths.

In exemplary optical fibers according to the present invention, co-doping serves to control losses in the transmitted optical signal and thus to limit bandwidth variation at transmission wavelengths of 850 nanometers and 1300 nanometers.

The co-doping is performed using fluorine together with another dopant (i.e., a dopant element) that increases the refractive index, such as germanium. In particular, exemplary co-doped optical fibers according to the present invention make it possible to obtain a high bandwidth at a transmission wavelength of 850 nanometers and a bandwidth that is only slightly degraded at a transmission wavelength of 1300 nanometers.

Appropriate concentrations of fluorine facilitate the simultaneous achievement of suitable bandwidths at 1300 nanometers and suitable attenuation values at 850 nanometers and 1300 nanometers.

By way of illustration, FIG. 5 depicts dopant concentration profiles in a prophetic exemplary optical fiber according to the present invention. The concentration profiles shown in FIG. 5 also serve to obtain the refractive index profile of FIG. 2. In FIG. 5, the germanium concentration curve is represented by a continuous line and the fluorine concentration curve is represented by a dashed line.

As shown in FIG. 5, for radial positions less than the central core's outer radius r₁, the fluorine concentration has a value of substantially zero in the center of the central core and increases progressively up to the end of the alpha profile of the central core.

In an exemplary embodiment, the fluorine concentration increases progressively with a parabolic shape. The fluorine concentration is expressed as the atomic ratio C_(F) between the fluorine and all of the constituents of the doped matrix of the central core (i.e., the doped core matrix). The terms “doped matrix” and “doped core matrix” are used to identify the assembly including (i) the base material of the central core (e.g., the core matrix, which may be silica), and (ii) all of the other constituents of the central core, including the dopant elements that are included in the doped matrix. The atomic ratio C_(F) may also be defined as the number of fluorine atoms divided by the total number of atoms (all constituents included) in the doped matrix of the central core (i.e., the doped core matrix).

In exemplary embodiments, the atomic ratio C_(F) of fluorine to all of the constituents of the doped core matrix is a function C_(F)(r) of the radial distance r from the center of the optical fiber (e.g., the center of the central core) and C_(F)(r) is defined by the following equation:

${{C_{F}(r)} = {{C_{F}^{(1)} + {{\left( \frac{r}{r_{1}} \right)^{p} \cdot \left( {C_{F}^{(2)} - C_{F}^{(1)}} \right)}{C_{F}(r)}}} = {C_{F}^{(1)} + {\left( \frac{r}{r_{1}} \right)^{p} \cdot \left( {C_{F}^{(2)} - C_{F}^{(1)}} \right)}}}};$

where:

p is a constant that is substantially equal to 2;

C_(F) ⁽²⁾ is the value of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile; and

C_(F) ⁽¹⁾ is a positive constant.

Typically, the value of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile C_(F) ⁽²⁾ is written as “C_(F,MAX)” (i.e., C_(F) ⁽²⁾=C_(F,MAX)). The value of C_(F,MAX) is typically between about 8.5×10⁻³ and 57×10⁻³. Furthermore, C_(F) ⁽¹⁾ is a positive constant, as noted, and typically has a value that is less than 1.25×10⁻³ (i.e., C_(F) ⁽¹⁾<1.25×10⁻³).

FIG. 6 graphically depicts the theoretical relationship between (i) the atomic ratio C_(F) of fluorine to all of the constituents of the doped core matrix and (ii) the refractive index difference attributable to the fluorine doping Δn_(F). In this regard, Δn_(F) is the refractive index difference at a wavelength of 633 nanometers between the refractive index of the core matrix and the refractive index of the core matrix doped solely with fluorine having the atomic ratio C_(F). This relationship is given by the following equation: Δn _(F)=−0.35×C _(F). For example, FIG. 6 illustrates that a fluorine atomic ratio C_(F) of 10×10⁻³ produces a refractive index difference Δn_(F) of −3.5×10⁻³.

Typically, the central core also includes another dopant that increases the refractive index (i.e., a refractive-index-increasing dopant). The concentration of this dopant that increases the refractive index in the central core is a function of (i) the central core's alpha-profile and (ii) the fluorine concentration in the central core. For example, while fabricating the optical fiber, the concentration of the dopant that increases the refractive index is obtained by iteration starting from the specified fluorine concentration and the desired alpha profile. An exemplary dopant that increases the refractive index is germanium.

The characteristics of the co-doping with fluorine and the dopant that increases the refractive index ensure a high bandwidth at a wavelength of 850 nanometers and a bandwidth that is only slightly degraded at a wavelength of 1300 nanometers.

In this regard, the co-doping characteristics facilitate the simultaneous achievement of (i) at a wavelength of 850 nanometers, an overfilled launch (OFL) bandwidth of 1500 MHz·km or greater, such as 3500 MHz·km or greater (e.g., 10,000 MHz·km or greater) and (ii) at a wavelength of 1300 nanometers, an overfilled launch (OFL) bandwidth of 500 MHz·km or greater (e.g., 600 MHz·km or greater).

As noted, exemplary optical fibers in accordance with the present invention include an inner cladding positioned between the central core and the outer optical cladding (e.g., immediately surrounding the central core). The inner cladding has (i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive index difference Δn₂ with respect to the outer optical cladding.

In exemplary embodiments, the optical fiber's inner cladding has a width w₂ of between 0.5 micron and 2 microns. The inner cladding's refractive index difference Δn₂ is typically constant over its entire width w₂. The inner cladding's refractive index difference Δn₂ is typically between −0.2×10⁻³ and 2×10⁻³.

The characteristics of the inner cladding's refractive index profile facilitate a reduction in cladding effect and contribute to obtaining suitable bandwidths at wavelengths of 850 nanometers and 1300 nanometers.

Exemplary optical fibers in accordance with the present invention also include a buried trench positioned between the inner cladding and the outer optical cladding (e.g., immediately surrounding the inner cladding). The buried trench has (i) an outer radius r₃, (ii) a width w₃, and (iii) a refractive index difference Δn₃ with respect to the outer optical cladding.

Typically, the term “buried trench” is used to describe a radial portion of an optical fiber that has a refractive index that is substantially less than the refractive index of the outer optical cladding. The buried trench's width w₃ is typically between 2 microns and 10 microns (e.g., between 3 microns and 5 microns). The buried trench's refractive index difference Δn₃ is typically less than −6×10⁻³. The buried trench's refractive index difference Δn₃ may be limited by the optical fiber fabrication process. For example, it may be difficult to achieve a buried trench having a refractive index difference Δn₃ lower than −15×10⁻³. Thus, in exemplary embodiments, the buried trench's refractive index difference Δn₃ is typically greater than −15×10⁻³.

The buried trench's refractive index difference Δn₃ is typically constant over the buried trench's width w₃. As noted, the inner cladding's refractive index difference Δn₂ is also typically constant over its entire width w₂. Exemplary optical fibers according to the invention, however, may have one or more refractive index differences that vary as a function of radial position (e.g., a trapezoidal, triangular, or alpha profile). For cladding layers having non-constant refractive indices, the respective refractive index differences (e.g., the inner cladding's refractive index difference Δn₂ and the buried trench's refractive index difference Δn₃) refer to the largest refractive index difference between the cladding layer and the outer optical cladding layer in terms of absolute value.

Furthermore, those of ordinary skill in the art will recognize that the outer optical cladding typically has a constant refractive index. That said, if the outer optical cladding has a non-constant refractive index, refractive index differences are typically measured with respect to the innermost portion of the outer optical cladding (i.e., that portion of the outer cladding that is closest to the central core and that may affect the propagation of optical signals within the optical fiber).

The characteristics of the buried trench's refractive index profile facilitate the achievement of low bending losses. In particular, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, exemplary optical fibers have bending losses of less than 0.2 dB (e.g., less than 0.1 dB). More typically, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, exemplary optical fibers have bending losses of less than 0.05 dB (e.g., less than 0.01 dB).

Additionally, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 1300 nanometers, exemplary optical fibers have bending losses of less than about 0.5 dB (e.g., less than about 0.3 dB). More typically, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 1300 nanometers, exemplary optical fibers have bending losses of less than 0.2 dB, such as less than 0.1 dB (e.g., less than 0.05 dB).

The bending losses values correspond to the launching conditions defined in the IEC 61280-4-1 standard, which is hereby incorporated by reference in its entirety. The optical fiber providing appropriate launching conditions is a conventional multimode optical fiber (MMF) having a numerical aperture of 0.2 and a core size of 50.0 microns±0.7 micron in accordance with the IEC 61280-4-1 standard.

The value of the bandwidth at a wavelength of 1300 nanometers is also a function of the characteristics of the buried trench. This can be better understood by referring to

FIGS. 7A and 7B. FIG. 7A graphically depicts the overfilled launch (OFL) bandwidth at a wavelength of 1300 nanometers (“OFL-BW @ 1300 nm”) as a function of the overfilled launch (OFL) bandwidth at a wavelength of 850 nanometers (“OFL-BW @ 850 nm”) for prophetic comparative optical fibers. FIG. 7B graphically depicts the overfilled launch (OFL) bandwidth at a wavelength of 1300 nanometers (“OFL-BW 1300 nm”) as a function of the overfilled launch (OFL) bandwidth at a wavelength of 850 nanometers (“OFL-BW 850 nm”) for exemplary optical fibers. The values of these bandwidths are expressed in MHz·km.

Curves 1-5 in FIG. 7A were acquired for prophetic comparative optical fibers having concentration profiles substantially the same as those shown in FIG. 3, with the exception of the profile of the buried trench.

Each optical fiber had a fluorine concentration at a different level in the buried trench, making it possible to achieve a refractive index difference Δn₃ for the corresponding buried trench. Thus, curves 1-5 were acquired, respectively, for buried trench refractive index differences Δn₃ of −5×10⁻³, −6×10⁻³, −8×10⁻³, −11×10⁻³, and −15×10⁻³. Each buried trench had a width w₃ of 4 microns.

Each curve is approximated from 21 points obtained for respective values of the parameter alpha. The parameter alpha varies over the range 1.9 to 2.1.

In each of the curves 1-5, for a given value of the parameter alpha, there is a corresponding pair of OFL bandwidth at a wavelength of 850 nanometers and OFL bandwidth at a wavelength of 1300 nanometers.

The maximum values of the OFL bandwidth at a wavelength of 850 nanometers and of the OFL bandwidth at a wavelength of 1300 nanometers are not obtained for the same value of the parameter alpha. For example, in curve 2, the maximum value of the OFL bandwidth at a wavelength of 850 nanometers is obtained at point A, whereas the maximum value of the OFL bandwidth at a wavelength of 1300 nanometers is obtained at point B.

In certain applications, it is desirable to have an alpha parameter value at which the OFL bandwidth at a wavelength of 850 nanometers is at a maximum. A comparison of curves 1-5 shows that at the point where the OFL bandwidth at a wavelength of 850 nanometers is at a maximum, the OFL bandwidth at a wavelength of 1300 nanometers increases with decreasing refractive index difference Δn₃ for the buried trench (i.e., as the buried trench deepens). Thus, in an optical fiber that has a buried trench, for a given width of trench, the bandwidth at a wavelength of 1300 nanometers depends on the refractive index difference Δn₃ of the buried trench.

The curves of FIG. 7B were obtained for exemplary optical fibers in accordance with the present invention having a refractive index difference Δn₃ for the buried trench of approximately −7×10⁻³ on the basis of measurements that are represented by points on the graph: the diamond-shaped points were obtained for C_(F,MAX)=8.5 and the triangle-shaped points for C_(F,MAX)=20. In this regard, the data points (i.e., the triangles and diamonds) were measured on exemplary optical fibers, while the curves represent theoretical models based on the data points.

In the optical fiber according to the present invention, co-doping with fluorine and with a dopant that increases the refractive index makes it possible to decrease the effect of the trench on the bandwidth at a wavelength of 1300 nanometers. This can be better understood by referring to FIG. 8.

FIG. 8 graphically depicts the prophetic overfilled launch (OFL) bandwidth at a wavelength of 1300 nanometers (“OFL-BW @ 1300 nm” expressed in MHz·km) as a function of the value C_(F,MAX) of the atomic ratio of fluorine to all of the constituents of the doped core matrix at the end of the central core's alpha-profile.

Curves 10, 11, and 12 in FIG. 8 correspond to exemplary optical fibers in accordance with the present invention including buried trenches having widths w₃ of 4 microns and respective refractive index differences Δn₃ of −4×10⁻³, −8×10⁻³, and −13×10⁻³.

As shown in FIG. 8, a value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile of between 8.5×10⁻³ to 57×10⁻³ facilitates the mitigation of the buried trench effect and an increase the OFL bandwidth at a wavelength of 1300 nanometers. A C_(F,MAX) value of less than 8.5×10⁻³ corresponds to a fluorine-induced refractive index difference of −3×10⁻³ or greater (See FIG. 6) and to optical fibers that are conventional. On the other hand, a C_(F,MAX) value of greater than 57×10⁻³ corresponds to a fluorine-induced refractive index difference of −20×10⁻³, which may be considered a lower limit for exemplary practical embodiments according to the present invention.

Unlike a modification to the value of the alpha-profile's alpha parameter α, the increase in fluorine concentration serves to broaden the bandwidth at a wavelength of 1300 nanometers without degrading the bandwidth at 850 nanometers.

Thus, in FIG. 8, curve 12 shows that increasing the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix of the central core at the end of the central core's alpha profile causes the bandwidth at a wavelength of 1300 nanometers to pass from less than 500 MHz·km to a bandwidth of about 700 MHz·km.

Additionally, in FIG. 8, curve 11 shows that increasing the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile causes the bandwidth at a wavelength of 1300 nanometers to pass from less than 500 MHz·km to a bandwidth greater than 1000 MHz·km.

Finally, in FIG. 8, curve 10 shows that increasing the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile causes the bandwidth at a wavelength of 1300 nanometers to pass from less than 1000 MHz·km to a bandwidth greater than 2500 MHz·km.

Without being bound to any particular theory, the present inventors have found that the improvement in the bandwidth at a wavelength of 1300 nanometers is also a function of the buried trench's refractive index difference Δn₃. FIG. 9 depicts this relationship.

FIG. 9 graphically depicts the prophetic overfilled launch (OFL) bandwidth at a wavelength of 1300 nanometers as a function of (i) the value C_(F,MAX) of the atomic ratio of fluorine to all of the constituents of the doped core matrix at the end of the central core's alpha-profile and (ii) the buried trench's refractive index difference Δn₃. As depicted, for a given pair of values Δn₃ and C_(F,MAX), an OFL bandwidth value at a wavelength of 1300 nanometers is obtained.

In an exemplary embodiment, the buried trench's refractive index difference Δn₃ and the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile satisfy the following inequality: C _(F,MAX)×1000>−8.97×1000×Δn ₃−61.54.

In other words, in this exemplary embodiment, the buried trench's refractive index difference Δn₃ and the refractive index difference Δn_(F,MAX) that results from the fluorine doping at the end of the central core's alpha profile satisfy the following inequality: Δn _(F,MAX)×1000<3.14×1000×Δn ₃+21.54.

The characteristics of these exemplary optical fibers, and in particular the relationships set out above, serve to obtain low bending losses, a high bandwidth at a wavelength of 850 nanometers, and a bandwidth greater than 500 MHz·km at a wavelength of 1300 nanometers.

In an exemplary embodiment having a buried trench with a refractive index difference Δn₃ of −7×10⁻³, the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile is greater than 1.25×10⁻³ and the refractive index difference Δn_(F,MAX) that results from the fluorine doping at the end of the central core's alpha profile is lower than −0.44×10⁻³.

In this exemplary embodiment, the refractive index difference Δn₃ of the buried trench and the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped matrix of the central core at the end of the alpha profile of the central core satisfy the following relationship: C _(F,MAX)×1000>−8.28×1000×Δn ₃−48.28.

In other words, in this exemplary embodiment, the refractive index difference Δn₃ of the buried trench and the refractive index difference Δn_(F,MAX) that results from the fluorine doping at the end of the central core's alpha profile satisfy the following relationship: Δn _(F,MAX)×1000<2.9×1000×Δn ₃+16.9.

The characteristics of these exemplary optical fibers according to the present invention, and in particular the relationships set out above, serve to obtain low bending losses, a high bandwidth at a wavelength of 850 nanometers, and a bandwidth greater than 550 MHz·km at a wavelength of 1300 nanometer.

In another exemplary embodiment having a buried trench with a refractive index difference Δn₃ of −7×10⁻³, the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile is greater than 9.7×10⁻³, and the refractive index difference Δn_(F,MAX) that results from the fluorine doping at the end of the central core's alpha profile is lower than −3.4×10⁻³.

For example, for a buried trench refractive index difference Δn₃ of −10×10⁻³, the refractive index difference Δn_(F,MAX) that results from the fluorine doping at the end of the central core's alpha profile is lower than −12.1×10⁻³.

In exemplary embodiments, the buried trench's refractive index difference Δn₃ and the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile satisfy the following relationship: C _(F,MAX)×1000>−8×1000×Δn ₃−42.28.

In other words, in these exemplary embodiments, the buried trench's refractive index difference Δn₃ and the refractive index difference Δn_(F,MAX) that results from the fluorine doping at the end of the central core's alpha profile satisfy the following relationship: Δn _(F,MAX)×1000<2.8×1000×Δn ₃+14.8.

The characteristics of these exemplary optical fibers according to the present invention, and in particular the relationships set out above, serve to obtain low bending losses, a high bandwidth at a wavelength 850 of nanometers, and a bandwidth greater than 600 MHz·km at a wavelength of 1300 nanometers.

In another exemplary embodiment having a buried trench with a refractive index difference Δn₃ of −7×10⁻³, the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile is greater than 13.7×10⁻³, and the refractive index difference Δn_(F,MAX) that results from the fluorine doping at the end of the central core's alpha profile is lower than −4.8×10⁻³.

In yet another exemplary embodiment, the buried trench has a refractive index difference Δn₃ of −10×10⁻³, the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha profile is greater than 37.7×10⁻³, and the refractive index difference Δn_(F,MAX) that results from the fluorine doping at the end of the central core's alpha profile is lower than −13.2×10⁻³.

FIG. 10 graphically depicts the desired alpha parameter α for achieving a maximum OFL bandwidth at a wavelength of 850 nanometers as a function of the value C_(F,MAX) of the atomic ratio of fluorine to all of the constituents of the doped core matrix at the end of the central core's alpha-profile.

The alpha parameters correspond to prophetic optical fibers having a buried trench with a width w₃ of 4 microns and different values of the refractive index difference Δn₃ relative to the outer optical cladding, namely: −13×10⁻³; −8×10⁻³; −4×10⁻³; and −1×10⁻³.

Without being bound to any particular theory, the present inventors have found that the depth of the buried trench does not affect the optimum target value of the alpha parameter, unlike the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha-profile.

In FIG. 10, certain representative points are superposed on one another. In exemplary optical fibers in accordance with the present invention, the value C_(F,MAX) of the atomic ratio between the fluorine and all of the constituents of the doped core matrix at the end of the central core's alpha-profile makes it possible to conserve an alpha parameter of between 1.9 and 2.1.

FIG. 11 graphically depicts the prophetic attenuation at a wavelength of 850 nanometers as a function of the value C_(F,MAX) of the atomic ratio of fluorine to all of the constituents of the doped core matrix at the end of the central core's alpha-profile.

As shown, in exemplary optical fibers, value C_(F,MAX) of the atomic ratio of fluorine to all of the constituents of the doped core matrix at the end of the central core's alpha-profile limits attenuation to between about 2.2 dB/km and 2.45 dB/km.

Thus, the characteristics of exemplary optical fibers according to the present invention, and in particular the relationship between the inner cladding's refractive index difference Δn₂ and the buried trench's refractive index difference Δn₃ enable low bending losses to be obtained while conserving a high bandwidth at a wavelength of 850 nanometers with a reduced cladding effect.

In another exemplary embodiment, the central core's alpha-index profile (i.e., the alpha-profile) is interrupted at a radius r_(alpha) and at a refractive index value Δn_(end) that is positive. In exemplary embodiments having an interrupted alpha-index profile, the radius r_(alpha) is less than outer radius r₁. In this embodiment, the outer radius r₁ is the radial distance at which the alpha-index profile of the central core would reach the value zero if it were not interrupted at the refractive index difference value Δn_(end). In other words, the outer radius r₁ is a theoretical radius used to define the shape of the alpha-index profile corresponding to the radial distance at which the refractive index difference between the central core and the outer optical cladding would be zero if the alpha-index profile were not interrupted. For radial positions between the radius r_(alpha) and the outer radius r₁, the refractive index difference is equal to the value Δn_(end).

In this interrupted alpha-index core exemplary embodiment, the inner cladding typically has a refractive index difference Δn₂ relative to the outer optical cladding that is equal to the refractive index difference Δn_(end) at the end of the alpha-profile (i.e., Δn₂=Δn_(end)). The refractive index difference Δn₂ is constant over the inner cladding and may be between about 0.2×10⁻³ and 2×10⁻³. The inner cladding's width w₂ is typically between about 0.5 micron and 2 microns.

In some exemplary embodiments, the inner cladding's refractive index difference Δn₂ and the buried trench's refractive index difference Δn₃ satisfy the following inequalities: −11.9×(1000×Δn ₂)²−3.4×(1000×Δn ₂)−7.2<1000×Δn ₃; and 1000×Δn ₃<−17.2×(1000×Δn ₂)²+16.5×(1000×Δn ₂)−8.0.

In yet another exemplary embodiment, the refractive index difference Δn_(end) at the end of the central core's alpha-index profile is between about −1×10⁻³ and zero.

In this exemplary embodiment, the inner cladding's refractive index difference Δn₂ and the buried trench's refractive index difference Δn₃ satisfy the following inequality: 1000×Δn ₃<−1.29×(1000×Δn ₂)²−1.64×1000×Δn ₂−2.51.

In yet another exemplary embodiment, the refractive index difference Δn_(end) at the end of the central core's alpha-index profile is between about −1×10⁻³ and zero, the inner cladding's refractive index difference Δn₂ is between about −0.6×10⁻³ and 2×10⁻³, and the inner cladding's width w₂ is between 0.5 micron and 2 microns.

In yet another exemplary embodiment, the inner cladding's refractive index difference Δn₂, the inner cladding's width w₂, and the buried trench's refractive index difference Δn₃ satisfy the following inequalities: 1000×Δn ₃ >A×(1000×Δn ₂)² +B×(1000×Δn ₂)+C, and 1000×Δn ₃ <D×(1000×Δn ₂)² +E×(1000×Δn ₂)+F, where:

-   -   A=182.29×w₂ ³−550×w₂ ²+545×w₂−177.32;     -   B=−135.42×w₂ ³+402×w₂ ²−399×w₂+130.22;     -   C=15×w₂ ³−57.25×w₂ ²+60.9×w₂−25.37;     -   D=−83.137×w₂ ³+235.06×w₂ ²−219.52×w₂+66.66;     -   E=129.56×w₂ ³−388.67×w₂ ²+378.13×w₂−121.11; and     -   F=−48.28×w₂ ³+137.84×w₂ ²−129.9×w₂+38.97.

Thus, exemplary optical fibers in accordance with the present invention, exhibit, at a wavelength of 850 nanometers, an OFL bandwidth greater than 1500 MHz·km, or even greater than 6000 MHz·km (e.g., greater than 10,000 MHz·km), while conserving a broad bandwidth at a wavelength of 1300 nanometers and low bending losses.

* * *

The OFL bandwidth is not the only parameter that can be used for assessing the suitability of an optical fiber in a broadband application. To improve the performance of an optical fiber for a broadband application, it is typically desirable to limit the cladding effect in the optical fiber at the interface between the central core and the inner cladding.

The dispersion mode delay acquired with a mask covering the outer portion of the optical fiber's central core, written DMDext and corresponding to the term “DMD value on the outer mask 0-23 μm” as defined in the standard FOTP-220, is an effective tool for characterizing the cladding effect of an optical fiber. The DMDext originates from a plot for DMD measured over 750 meters of optical fiber. The light source used may be a pulsed Ti:Sapphire laser emitting at 850 nanometers. The source emits pulses of less than 40 ps at quarter height, and the RMS (Root Mean Square) spectral width is less than 0.1 nanometer.

An exemplary optical fiber according to the present invention exhibits improved DMDext values at a wavelength of 850 nanometers, while conserving a broad bandwidth at a wavelength of 1300 nanometers and low bending losses.

The characteristics of exemplary optical fibers according to the present invention enable DMDext to be decreased compared with conventional optical fibers. In particular, at a wavelength of 850 nanometers, exemplary optical fibers of the invention typically exhibit a DMDext value of less than 0.33 ps/m (e.g., less 0.25 ps/m). More typically, at a wavelength of 850 nanometers, the exemplary optical fiber exhibits an outer DMD value of 0.14 ps/m or less.

In some exemplary embodiments, the optical fiber of the present invention is compatible with the ITU-T standard G.651.1. Thus, the optical fiber includes a 50-micron central core (i.e., a central core diameter of 50±3 microns or outer radius of 25±1.5 microns) and a numerical aperture of 0.2±0.015.

The invention also provides a multimode optical system including at least a portion of fiber in accordance with one or more of the foregoing exemplary embodiments of an optical fiber in accordance with the present invention. In particular, the optical fiber may present a bit rate greater than or equal to 10 Gb/s over 100 meters. The optical system may also present a rate greater than or equal to 10 Gb/s over 300 meters.

The optical fiber in accordance with the invention may be used to obtain low bending losses and a broad bandwidth at the transmission wavelengths of 850 nanometers and 1300 nanometers, using a high volume and low cost fabrication process.

The optical fiber of the invention may be installed in numerous transmission systems with good compatibility with the other optical fibers of the system.

* * *

The present optical fibers may facilitate the reduction in overall optical-fiber diameter. As will be appreciated by those having ordinary skill in the art, a reduced-diameter optical fiber is cost-effective, requiring less raw material. Moreover, a reduced-diameter optical fiber requires less deployment space (e.g., within a buffer tube and/or fiber optic cable), thereby facilitating increased fiber count and/or reduced cable size.

Those having ordinary skill in the art will recognize that an optical fiber with a primary coating (and an optional secondary coating and/or ink layer) typically has an outer diameter of between about 235 microns and about 265 microns (μm). The component glass fiber itself (i.e., the glass core and surrounding cladding layers) typically has a diameter of about 125 microns, such that the total coating thickness is typically between about 55 microns and 70 microns.

With respect to the present optical fiber, the component glass fiber typically has an outer diameter of about 125 microns. With respect to the optical fiber's surrounding coating layers, the primary coating typically has an outer diameter of between about 175 microns and about 195 microns (i.e., a primary coating thickness of between about 25 microns and 35 microns), and the secondary coating typically has an outer diameter of between about 235 microns and about 265 microns (i.e., a secondary coating thickness of between about 20 microns and 45 microns). Optionally, the present optical fiber may include an outermost ink layer, which is typically between two and ten microns in thickness.

In one alternative embodiment, an optical fiber may possess a reduced diameter (e.g., an outermost diameter between about 150 microns and 230 microns). In this alternative optical fiber configuration, the thickness of the primary coating and/or secondary coating is reduced, while the diameter of the component glass fiber is maintained at about 125 microns. (Those having ordinary skill in the art will appreciate that, unless otherwise specified, diameter measurements refer to outer diameters.)

By way of illustration, in such exemplary embodiments, the primary coating layer may have an outer diameter of between about 135 microns and about 175 microns (e.g., about 160 microns), typically less than 165 microns (e.g., between about 135 microns and 150 microns), and usually more than 140 microns (e.g., between about 145 microns and 155 microns, such as about 150 microns).

Moreover, in such exemplary embodiments, the secondary coating layer may have an outer diameter of between about 150 microns and about 230 microns (e.g., more than about 165 microns, such as 190-210 microns or so), typically between about 180 microns and 200 microns. In other words, the total diameter of the optical fiber is reduced to less than about 230 microns (e.g., between about 195 microns and 205 microns, and especially about 200 microns). By way of further illustration, an optical fiber may employ a secondary coating of about 197 microns at a tolerance of +/−5 microns (i.e., a secondary-coating outer diameter of between 192 microns to 202 microns). Typically, the secondary coating will retain a thickness of at least about 10 microns (e.g., an optical fiber having a reduced thickness secondary coating of between 15 microns and 25 microns).

In another alternative embodiment, the outer diameter of the component glass fiber may be reduced to less than 125 microns (e.g., between about 60 microns and 120 microns), perhaps between about 70 microns and 115 microns (e.g., about 80-110 microns). This may be achieved, for instance, by reducing the thickness of one or more cladding layers. As compared with the prior alternative embodiment, (i) the total diameter of the optical fiber may be reduced (i.e., the thickness of the primary and secondary coatings are maintained in accordance with the prior alternative embodiment) or (ii) the respective thicknesses of the primary and/or secondary coatings may be increased relative to the prior alternative embodiment (e.g., such that the total diameter of the optical fiber might be maintained).

By way of illustration, with respect to the former, a component glass fiber having a diameter of between about 90 and 100 microns might be combined with a primary coating layer having an outer diameter of between about 110 microns and 150 microns (e.g., about 125 microns) and a secondary coating layer having an outer diameter of between about 130 microns and 190 microns (e.g., about 155 microns). With respect to the latter, a component glass fiber having a diameter of between about 90 and 100 microns might be combined with a primary coating layer having an outer diameter of between about 120 microns and 140 microns (e.g., about 130 microns) and a secondary coating layer having an outer diameter of between about 160 microns and 230 microns (e.g., about 195-200 microns).

Reducing the diameter of the component glass fiber might make the resulting optical fiber more susceptible to microbending attenuation. That said, the advantages of further reducing optical-fiber diameter might be worthwhile for some optical-fiber applications.

As noted, the present optical fibers may include one or more coating layers (e.g., a primary coating and a secondary coating). At least one of the coating layers—typically the secondary coating—may be colored and/or possess other markings to help identify individual fibers. Alternatively, a tertiary ink layer may surround the primary and secondary coatings.

The present optical fibers may be manufactured by drawing from final preforms.

A final preform may be manufactured by providing a primary preform with an outer overcladding layer (i.e., an overcladding process). The outer overcladding layer typically consists of doped or undoped, natural or synthetic, silica glass. Several methods are available for providing the outer overcladding layer.

In a first exemplary method, the outer overcladding layer may be provided by depositing and vitrifying natural or synthetic silica particles on the outer periphery of the primary preform under the influence of heat. Such a process is known, for example, from U.S. Pat. Nos. 5,522,007, 5,194,714, 6,269,663, and 6,202,447, each of which is hereby incorporated by reference in its entirety.

In another exemplary method, a primary preform may be overcladded using a silica sleeve tube, which may or may not be doped. This sleeve tube may then be collapsed onto the primary preform.

In yet another exemplary method, an overcladding layer may be applied via an Outside Vapor Deposition (OVD) method. Here, a soot layer is first deposited on the outer periphery of a primary preform, and then the soot layer is vitrified to form glass.

The primary preforms may be manufactured via outside vapor deposition techniques, such as Outside Vapor Deposition (OVD) and Vapor Axial Deposition (VAD). Alternatively, the primary preforms may be manufactured via inside deposition techniques in which glass layers are deposited on the inner surface of a substrate tube of doped or undoped silica glass, such as Modified Chemical Vapor Deposition (MCVD), Furnace Chemical Vapor Deposition (FCVD), and Plasma Chemical Vapor Deposition (PCVD).

By way of example, the primary preforms may be manufactured using a PCVD process, which can precisely control the central core's gradient refractive index profile.

A depressed trench, for instance, may be deposited on the inner surface of a substrate tube as part of the chemical vapor deposition process. More typically, a depressed trench may be manufactured either (i) by using a fluorine-doped substrate tube as the starting point of the internal deposition process for deposition of the gradient refractive index central core or (ii) by sleeving a fluorine-doped silica tube over the gradient refractive index central core, which itself may be produced using an outside deposition process (e.g., OVD or VAD). Accordingly, a component glass fiber manufactured from the resulting preform may have a depressed trench located at the periphery of its central core.

As noted, a primary preform may be manufactured via an inside deposition process using a fluorine-doped substrate tube. The resulting tube containing the deposited layers may be sleeved by one or more additional fluorine-doped silica tubes so as to increase the thickness of a depressed trench, or to create a depressed trench having a varying refractive index over its width. Although not required, one or more additional sleeve tubes (e.g., fluorine-doped substrate tubes) may be collapsed onto the primary preform before an overcladding step is carried out. The process of sleeving and collapsing is sometimes referred to as jacketing and may be repeated to build several glass layers on the outside of the primary preform.

* * *

The present optical fibers may be deployed in various structures, such as those exemplary structures disclosed hereinafter.

For example, one or more of the present optical fibers may be enclosed within a buffer tube. For instance, optical fiber may be deployed in either a single-fiber loose buffer tube or a multi-fiber loose buffer tube. With respect to the latter, multiple optical fibers may be bundled or stranded within a buffer tube or other structure. In this regard, within a multi-fiber loose buffer tube, fiber sub-bundles may be separated with binders (e.g., each fiber sub-bundle is enveloped in a binder). Moreover, fan-out tubing may be installed at the termination of such loose buffer tubes to directly terminate loose buffered optical fibers with field-installed connectors.

In other embodiments, the buffer tube may tightly surround the outermost optical fiber coating (i.e., tight buffered fiber) or otherwise surround the outermost optical-fiber coating or ink layer to provide an exemplary radial clearance of between about 50 and 100 microns (i.e., a semi-tight buffered fiber).

With respect to the former tight buffered fiber, the buffering may be formed by coating the optical fiber with a curable composition (e.g., a UV-curable material) or a thermoplastic material. The outer diameter of tight buffer tubes, regardless of whether the buffer tube is formed from a curable or non-curable material, is typically less than about 1,000 microns (e.g., either about 500 microns or about 900 microns).

With respect to the latter semi-tight buffered fiber, a lubricant may be included between the optical fiber and the buffer tube (e.g., to provide a gliding layer).

As will be known by those having ordinary skill in the art, an exemplary buffer tube enclosing optical fibers as disclosed herein may be formed of polyolefins (e.g., polyethylene or polypropylene), including fluorinated polyolefins, polyesters (e.g., polybutylene terephthalate), polyamides (e.g., nylon), as well as other polymeric materials and blends. In general, a buffer tube may be formed of one or more layers. The layers may be homogeneous or include mixtures or blends of various materials within each layer.

In this context, the buffer tube may be extruded (e.g., an extruded polymeric material) or pultruded (e.g., a pultruded, fiber-reinforced plastic). By way of example, the buffer tube may include a material to provide high temperature and chemical resistance (e.g., an aromatic material or polysulfone material).

Although buffer tubes typically have a circular cross section, buffer tubes alternatively may have an irregular or non-circular shape (e.g., an oval or a trapezoidal cross-section).

Alternatively, one or more of the present optical fibers may simply be surrounded by an outer protective sheath or encapsulated within a sealed metal tube. In either structure, no intermediate buffer tube is necessarily required.

Multiple optical fibers as disclosed herein may be sandwiched, encapsulated, and/or edge bonded to form an optical fiber ribbon. Optical fiber ribbons can be divisible into subunits (e.g., a twelve-fiber ribbon that is splittable into six-fiber subunits). Moreover, a plurality of such optical fiber ribbons may be aggregated to form a ribbon stack, which can have various sizes and shapes.

For example, it is possible to form a rectangular ribbon stack or a ribbon stack in which the uppermost and lowermost optical fiber ribbons have fewer optical fibers than those toward the center of the stack. This construction may be useful to increase the density of optical elements (e.g., optical fibers) within the buffer tube and/or cable.

In general, it is desirable to increase the filling of transmission elements in buffer tubes or cables, subject to other constraints (e.g., cable or mid-span attenuation). The optical elements themselves may be designed for increased packing density. For example, the optical fiber may possess modified properties, such as improved refractive-index profile, core or cladding dimensions, or primary-coating thickness and/or modulus, to improve microbending and macrobending characteristics.

By way of example, a rectangular ribbon stack may be formed with or without a central twist (i.e., a “primary twist”). Those having ordinary skill in the art will appreciate that a ribbon stack is typically manufactured with rotational twist to allow the tube or cable to bend without placing excessive mechanical stress on the optical fibers during winding, installation, and use. In a structural variation, a twisted (or untwisted) rectangular ribbon stack may be further formed into a coil-like configuration (e.g., a helix) or a wave-like configuration (e.g., a sinusoid). In other words, the ribbon stack may possess regular “secondary” deformations.

As will be known to those having ordinary skill in the art, such optical fiber ribbons may be positioned within a buffer tube or other surrounding structure, such as a buffer-tube-free cable. Subject to certain restraints (e.g., attenuation), it is desirable to increase the density of elements such as optical fibers or optical fiber ribbons within buffer tubes and/or optical fiber cables.

A plurality of buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be positioned externally adjacent to and stranded around a central strength member. This stranding can be accomplished helically in one direction, known as “S” or “Z” stranding, or via Reverse Oscillated Lay stranding, known as “S-Z” stranding. Stranding about the central strength member reduces optical fiber strain when cable strain occurs during installation and use.

Those having ordinary skill in the art will understand the benefit of minimizing fiber strain for both tensile cable strain and longitudinal compressive cable strain during installation or operating conditions.

With respect to tensile cable strain, which may occur during installation, the cable will become longer while the optical fibers can migrate closer to the cable's neutral axis to reduce, if not eliminate, the strain being translated to the optical fibers. With respect to longitudinal compressive strain, which may occur at low operating temperatures due to shrinkage of the cable components, the optical fibers will migrate farther away from the cable's neutral axis to reduce, if not eliminate, the compressive strain being translated to the optical fibers.

In a variation, two or more substantially concentric layers of buffer tubes may be positioned around a central strength member. In a further variation, multiple stranding elements (e.g., multiple buffer tubes stranded around a strength member) may themselves be stranded around each other or around a primary central strength member.

Alternatively, a plurality of buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be simply placed externally adjacent to the central strength member (i.e., the buffer tubes are not intentionally stranded or arranged around the central strength member in a particular manner and run substantially parallel to the central strength member).

Alternatively still, the present optical fibers may be positioned within a central buffer tube (i.e., the central buffer tube cable has a central buffer tube rather than a central strength member). Such a central buffer tube cable may position strength members elsewhere. For instance, metallic or non-metallic (e.g., GRP) strength members may be positioned within the cable sheath itself, and/or one or more layers of high-strength yarns (e.g., aramid or non-aramid yarns) may be positioned parallel to or wrapped (e.g., contrahelically) around the central buffer tube (i.e., within the cable's interior space). As will be understood by those having ordinary skill in the art, such strength yarns provide tensile strength to fiber optic cables. Likewise, strength members can be included within the buffer tube's casing.

Strength yarns may be coated with a lubricant (e.g., fluoropolymers), which may reduce unwanted attenuation in fiber optic cables (e.g., rectangular, flat ribbon cables or round, loose tube cables) that are subjected to relatively tight bends (i.e., a low bend radius). Moreover, the presence of a lubricant on strength yarns (e.g., aramid strength yarns) may facilitate removal of the cable jacketing by reducing unwanted bonding between the strength yarns and the surrounding cable jacket.

In other embodiments, the optical fibers may be placed within a slotted core cable. In a slotted core cable, optical fibers, individually or as a fiber ribbon, may be placed within pre-shaped helical grooves (i.e., channels) on the surface of a central strength member, thereby forming a slotted core unit. The slotted core unit may be enclosed by a buffer tube. One or more of such slotted core units may be placed within a slotted core cable. For example, a plurality of slotted core units may be helically stranded around a central strength member.

Alternatively, the optical fibers may also be stranded in a maxitube cable design, whereby the optical fibers are stranded around themselves within a large multi-fiber loose buffer tube rather than around a central strength member. In other words, the large multi-fiber loose buffer tube is centrally positioned within the maxitube cable. For example, such maxitube cables may be deployed in optical ground wires (OPGW).

In another cabling embodiment, multiple buffer tubes may be stranded around themselves without the presence of a central member. These stranded buffer tubes may be surrounded by a protective tube. The protective tube may serve as the outer casing of the fiber optic cable or may be further surrounded by an outer sheath. The protective tube may either tightly surround or loosely surround the stranded buffer tubes.

As will be known to those having ordinary skill in the art, additional elements may be included within a cable core. For example, copper cables or other active, transmission elements may be stranded or otherwise bundled within the cable sheath. Passive elements may also be placed within the cable core, such as between the interior walls of the buffer tubes and the enclosed optical fibers. Alternatively and by way of example, passive elements may be placed outside the buffer tubes between the respective exterior walls of the buffer tubes and the interior wall of the cable jacket, or within the interior space of a buffer-tube-free cable.

For example, yarns, nonwovens, fabrics (e.g., tapes), foams, or other materials containing water-swellable material and/or coated with water-swellable materials (e.g., including super absorbent polymers (SAPs), such as SAP powder) may be employed to provide water blocking and/or to couple the optical fibers to the surrounding buffer tube and/or cable jacketing (e.g., via adhesion, friction, and/or compression). Exemplary water-swellable elements are disclosed in commonly assigned U.S. Pat. No. 7,515,795 for a Water-Swellable Tape, Adhesive-Backed for Coupling When Used Inside a Buffer Tube, which is hereby incorporated by reference in its entirety.

Moreover, an adhesive (e.g., a hot-melt adhesive or curable adhesive, such as a silicone acrylate cross-linked by exposure to actinic radiation) may be provided on one or more passive elements (e.g., water-swellable material) to bond the elements to the buffer tube. An adhesive material may also be used to bond the water-swellable element to optical fibers within the buffer tube. Exemplary arrangements of such elements are disclosed in commonly assigned U.S. Pat. No. 7,599,589 for a Gel-Free Buffer Tube with Adhesively Coupled Optical Element, which is hereby incorporated by reference in its entirety.

The buffer tubes (or buffer-tube-free cables) may also contain a thixotropic composition (e.g., grease or grease-like gels) between the optical fibers and the interior walls of the buffer tubes. For example, filling the free space inside a buffer tube with water-blocking, petroleum-based filling grease helps to block the ingress of water. Further, the thixotropic filling grease mechanically (i.e., viscously) couples the optical fibers to the surrounding buffer tube.

Such thixotropic filling greases are relatively heavy and messy, thereby hindering connection and splicing operations. Thus, the present optical fibers may be deployed in dry cable structures (i.e., grease-free buffer tubes).

Exemplary buffer tube structures that are free from thixotropic filling greases are disclosed in commonly assigned U.S. Pat. No. 7,724,998 for a Coupling Composition for Optical Fiber Cables (Parris et al.), which is hereby incorporated by reference in its entirety. Such buffer tubes employ coupling compositions formed from a blend of high-molecular weight elastomeric polymers (e.g., about 35 weight percent or less) and oils (e.g., about 65 weight percent or more) that flow at low temperatures. Unlike thixotropic filling greases, the coupling composition (e.g., employed as a cohesive gel or foam) is typically dry and, therefore, less messy during splicing.

As will be understood by those having ordinary skill in the art, a cable enclosing optical fibers as disclosed herein may have a sheath formed from various materials in various designs. Cable sheathing may be formed from polymeric materials such as, for example, polyethylene, polypropylene, polyvinyl chloride (PVC), polyamides (e.g., nylon), polyester (e.g., PBT), fluorinated plastics (e.g., perfluorethylene propylene, polyvinyl fluoride, or polyvinylidene difluoride), and ethylene vinyl acetate. The sheath and/or buffer tube materials may also contain other additives, such as nucleating agents, flame-retardants, smoke-retardants, antioxidants, UV absorbers, and/or plasticizers.

The cable sheathing may be a single jacket formed from a dielectric material (e.g., non-conducting polymers), with or without supplemental structural components that may be used to improve the protection (e.g., from rodents) and strength provided by the cable sheath. For example, one or more layers of metallic (e.g., steel) tape, along with one or more dielectric jackets, may form the cable sheathing. Metallic or fiberglass reinforcing rods (e.g., GRP) may also be incorporated into the sheath. In addition, aramid, fiberglass, or polyester yarns may be employed under the various sheath materials (e.g., between the cable sheath and the cable core), and/or ripcords may be positioned, for example, within the cable sheath.

Similar to buffer tubes, optical fiber cable sheaths typically have a circular cross section, but cable sheaths alternatively may have an irregular or non-circular shape (e.g., an oval, trapezoidal, or flat cross-section).

By way of example, the present optical fiber may be incorporated into single-fiber drop cables, such as those employed for Multiple Dwelling Unit (MDU) applications. In such deployments, the cable jacketing must exhibit crush resistance, abrasion resistance, puncture resistance, thermal stability, and fire resistance as required by building codes. An exemplary material for such cable jackets is thermally stable, flame-retardant polyurethane (PUR), which mechanically protects the optical fibers yet is sufficiently flexible to facilitate easy MDU installations. Alternatively, a flame-retardant polyolefin or polyvinyl chloride sheath may be used.

In general, and as will be known to those having ordinary skill in the art, a strength member is typically in the form of a rod or braided/helically wound wires or fibers, though other configurations will be within the knowledge of those having ordinary skill in the art.

Optical fiber cables containing optical fibers as disclosed may be variously deployed, including as drop cables, distribution cables, feeder cables, trunk cables, and stub cables, each of which may have varying operational requirements (e.g., temperature range, crush resistance, UV resistance, and minimum bend radius).

Such optical fiber cables may be installed within ducts, microducts, plenums, or risers. By way of example, an optical fiber cable may be installed in an existing duct or microduct by pulling or blowing (e.g., using compressed air). An exemplary cable installation method is disclosed in commonly assigned U.S. Pat. No. 7,574,095 for a Communication Cable Assembly and Installation Method, (Lock et al.), and U.S. Pat. No. 7,665,902 for a Modified Pre-Ferrulized Communication Cable Assembly and Installation Method, (Griffioen et al.), each of which is incorporated by reference in its entirety.

As noted, buffer tubes containing optical fibers (e.g., loose or ribbonized fibers) may be stranded (e.g., around a central strength member). In such configurations, an optical fiber cable's protective outer sheath may have a textured outer surface that periodically varies lengthwise along the cable in a manner that replicates the stranded shape of the underlying buffer tubes. The textured profile of the protective outer sheath can improve the blowing performance of the optical fiber cable. The textured surface reduces the contact surface between the cable and the duct or microduct and increases the friction between the blowing medium (e.g., air) and the cable. The protective outer sheath may be made of a low coefficient-of-friction material, which can facilitate blown installation. Moreover, the protective outer sheath can be provided with a lubricant to further facilitate blown installation.

In general, to achieve satisfactory long-distance blowing performance (e.g., between about 3,000 to 5,000 feet or more), the outer cable diameter of an optical fiber cable should be no more than about 70 to 80 percent of the duct's or microduct's inner diameter.

Compressed air may also be used to install optical fibers in an air blown fiber system. In an air blown fiber system, a network of unfilled cables or microducts is installed prior to the installation of optical fibers. Optical fibers may subsequently be blown into the installed cables as necessary to support the network's varying requirements.

Moreover, the optical fiber cables may be directly buried in the ground or, as an aerial cable, suspended from a pole or pylori. An aerial cable may be self-supporting, or secured or lashed to a support (e.g., messenger wire or another cable). Exemplary aerial fiber optic cables include overhead ground wires (OPGW), all-dielectric self-supporting cables (ADSS), all dielectric lash cables (AD-Lash), and figure-eight cables, each of which is well understood by those having ordinary skill in the art. Figure-eight cables and other designs can be directly buried or installed into ducts, and may optionally include a toning element, such as a metallic wire, so that they can be found with a metal detector.

In addition, although the optical fibers may be further protected by an outer cable sheath, the optical fiber itself may be further reinforced so that the optical fiber may be included within a breakout cable, which allows for the individual routing of individual optical fibers.

To effectively employ the present optical fibers in a transmission system, connections are required at various points in the network. Optical fiber connections are typically made by fusion splicing, mechanical splicing, or mechanical connectors.

The mating ends of connectors can be installed to the optical fiber ends either in the field (e.g., at the network location) or in a factory prior to installation into the network. The ends of the connectors are mated in the field in order to connect the optical fibers together or connect the optical fibers to the passive or active components. For example, certain optical fiber cable assemblies (e.g., furcation assemblies) can separate and convey individual optical fibers from a multiple optical fiber cable to connectors in a protective manner.

The deployment of such optical fiber cables may include supplemental equipment, which itself may employ the present optical fiber as previously disclosed. For instance, an amplifier may be included to improve optical signals. Dispersion compensating modules may be installed to reduce the effects of chromatic dispersion and polarization mode dispersion. Splice boxes, pedestals, and distribution frames, which may be protected by an enclosure, may likewise be included. Additional elements include, for example, remote terminal switches, optical network units, optical splitters, and central office switches.

A cable containing the present optical fibers may be deployed for use in a communication system (e.g., networking or telecommunications). A communication system may include fiber optic cable architecture such as fiber-to-the-node (FTTN), fiber-to-the-telecommunications enclosure (FTTE), fiber-to-the-curb (FTTC), fiber-to-the-building (FTTB), and fiber-to-the-home (FTTH), as well as long-haul or metro architecture. Moreover, an optical module or a storage box that includes a housing may receive a wound portion of the optical fiber disclosed herein. By way of example, the optical fiber may be wound around a bending radius of less than about 15 millimeters (e.g., 10 millimeters or less, such as about 5 millimeters) in the optical module or the storage box.

Moreover, present optical fibers may be used in other applications, including, without limitation, fiber optic sensors or illumination applications (e.g., lighting).

* * *

The present optical fibers may include Fiber Bragg Grating (FBG). As will be known by those having ordinary skill in the art, FBG is a periodic or aperiodic variation in the refractive index of an optical fiber core and/or cladding. This variation in the refractive index results in a range of wavelengths (e.g., a narrow range) being reflected rather than transmitted, with maximum reflectivity occurring at the Bragg wavelength.

Fiber Bragg Grating is commonly written into an optical fiber by exposing the optical fiber to an intense source of ultraviolet light (e.g., a UV laser). In this respect, UV photons may have enough energy to break molecular bonds within an optical fiber, which alters the structure of the optical fiber, thereby increasing the optical fiber's refractive index. Moreover, dopants (e.g., boron or germanium) and/or hydrogen loading can be employed to increase photosensitivity.

In order to expose a coated glass fiber to UV light for the creation of FBG, the coating may be removed. Alternatively, coatings that are transparent at the particular UV wavelengths (e.g., the UV wavelengths emitted by a UV laser to write FBG) may be employed to render coating removal unnecessary. In addition, silicone, polyimide, acrylate, or PFCB coatings, for instance, may be employed for high-temperature applications.

A particular FBG pattern may be created by employing (i) a photomask placed between the UV light source and the optical fiber, (ii) interference between multiple UV light beams, which interfere with each other in accordance with the desired FBG pattern (e.g., a uniform, chirped, or titled pattern), or (iii) a narrow UV light beam for creating individual variations. The FBG structure may have, for example, a uniform positive-only index change, a Gaussian-apodized index change, a raised-cosine-apodized index change, or a discrete phase-shift index change. Multiple FBG patterns may be combined on a single optical fiber.

Optical fibers having FBG may be employed in various sensing applications (e.g., for detecting vibration, temperature, pressure, moisture, or movement). In this respect, changes in the optical fiber (e.g., a change in temperature) result in a shift in the Bragg wavelength, which is measured by a sensor. FBG may be used to identify a particular optical fiber (e.g., if the optical fiber is broken into pieces).

Fiber Bragg Grating may also be used in various active or passive communication components (e.g., wavelength-selective filters, multiplexers, demultiplexers, Mach-Zehnder interferometers, distributed Bragg reflector lasers, pump/laser stabilizers, and supervisory channels).

* * *

To supplement the present disclosure, this application incorporates entirely by reference the following commonly assigned patents, patent application publications, and patent applications: U.S. Pat. No. 4,838,643 for a Single Mode Bend Insensitive Fiber for Use in Fiber Optic Guidance Applications (Hodges et al.); U.S. Pat. No. 7,623,747 for a Single Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No. 7,587,111 for a Single-Mode Optical Fiber (de Montmorillon et al.); U.S. Pat. No. 7,356,234 for a Chromatic Dispersion Compensating Fiber (de Montmorillon et al.); U.S. Pat. No. 7,483,613 for a Chromatic Dispersion Compensating Fiber (Bigot-Astruc et al.); U.S. Pat. No. 7,526,177 for a Fluorine-Doped Optical Fiber (Matthijsse et al.); U.S. Pat. No. 7,555,186 for an Optical Fiber (Flammer et al.); U.S. Pat. No. 8,055,111 for a Dispersion-Shifted Optical Fiber (Sillard et al.); U.S. Pat. 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To supplement the present disclosure, this application further incorporates entirely by reference the following commonly assigned patents, patent application publications, and patent applications: U.S. Pat. No. 5,574,816 for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical Fiber Cables and Method for Making the Same; U.S. Pat. No. 5,717,805 for Stress Concentrations in an Optical Fiber Ribbon to Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 5,761,362 for Polypropylene-Polyethylene Copolymer Buffer Tubes for Optical Fiber Cables and Method for Making the Same; U.S. Pat. No. 5,911,023 for Polyolefin Materials Suitable for Optical Fiber Cable Components; U.S. Pat. No. 5,982,968 for Stress Concentrations in an Optical Fiber Ribbon to Facilitate Separation of Ribbon Matrix Material; U.S. Pat. No. 6,035,087 for an Optical Unit for Fiber Optic Cables; U.S. Pat. No. 6,066,397 for Polypropylene Filler Rods for Optical Fiber Communications Cables; U.S. Pat. No. 6,175,677 for an Optical Fiber Multi-Ribbon and Method for Making the Same; U.S. Pat. No. 6,085,009 for Water Blocking Gels Compatible with Polyolefin Optical Fiber Cable Buffer Tubes and Cables Made Therewith; U.S. Pat. No. 6,215,931 for Flexible Thermoplastic Polyolefin Elastomers for Buffering Transmission Elements in a Telecommunications Cable; U.S. Pat. No. 6,134,363 for a Method for Accessing Optical Fibers in the Midspan Region of an Optical Fiber Cable; U.S. Pat. No. 6,381,390 for a Color-Coded Optical Fiber Ribbon and Die for Making the Same; U.S. Pat. No. 6,181,857 for a Method for Accessing Optical Fibers Contained in a Sheath; U.S. Pat. No. 6,314,224 for a Thick-Walled Cable Jacket with Non-Circular Cavity Cross Section; U.S. Pat. No. 6,334,016 for an Optical Fiber Ribbon Matrix Material Having Optimal Handling Characteristics; U.S. Pat. No. 6,321,012 for an Optical Fiber Having Water Swellable Material for Identifying Grouping of Fiber Groups; U.S. Pat. No. 6,321,014 for a Method for Manufacturing Optical Fiber Ribbon; U.S. Pat. No. 6,210,802 for Polypropylene Filler Rods for Optical Fiber Communications Cables; U.S. Pat. No. 6,493,491 for an Optical Drop Cable for Aerial Installation; U.S. Pat. No. 7,346,244 for a Coated Central Strength Member for Fiber Optic Cables with Reduced Shrinkage; U.S. Pat. No. 6,658,184 for a Protective Skin for Optical Fibers; U.S. Pat. No. 6,603,908 for a Buffer Tube that Results in Easy Access to and Low Attenuation of Fibers Disposed Within Buffer Tube; U.S. Pat. No. 7,045,010 for an Applicator for High-Speed Gel Buffering of Flextube Optical Fiber Bundles; U.S. Pat. No. 6,749,446 for an Optical Fiber Cable with Cushion Members Protecting Optical Fiber Ribbon Stack; U.S. Pat. No. 6,922,515 for a Method and Apparatus to Reduce Variation of Excess Fiber Length in Buffer Tubes of Fiber Optic Cables; U.S. Pat. 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* * *

In the specification and/or figures, typical embodiments of the invention have been disclosed. The present invention is not limited to such exemplary embodiments. The use of the term “and/or” includes any and all combinations of one or more of the associated listed items. The figures are schematic representations and so are not necessarily drawn to scale. Unless otherwise noted, specific terms have been used in a generic and descriptive sense and not for purposes of limitation. 

The invention claimed is:
 1. An optical fiber, comprising: a central core surrounded by an outer optical cladding, said central core having an outer radius r₁ and an alpha-index profile with respect to said outer optical cladding, said central core comprising a core matrix doped with at least fluorine and a dopant element that increases refractive index; an inner cladding positioned between said central core and said outer cladding, said inner cladding having (i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive index difference Δn₂ with respect to said outer optical cladding; and a buried trench positioned between said inner cladding and said outer optical cladding, said buried trench having (i) an outer radius r₃, (ii) a width w₃ of between about 2 microns and 10 microns, and (iii) a refractive index difference Δn₃ with respect to said outer optical cladding of between about −15×10⁻³ and −6×10⁻³; wherein, within said central core, the atomic ratio C_(F) of fluorine to all of the constituents of said doped core matrix increases from the center of the optical fiber to said central core's outer radius r₁; and wherein, at said central core's outer radius r₁, the atomic ratio C_(F) of fluorine to all of the constituents of said doped core matrix has a maximum value C_(F,MAX) of between about 8.5×10⁻³ and 57×10⁻³.
 2. The optical fiber according to claim 1, wherein said core matrix is silica and said dopant element that increases refractive index is germanium.
 3. The optical fiber according to claim 1, wherein, at said central core's outer radius r₁, said buried trench's refractive index difference Δn₃ and the atomic ratio of fluorine's maximum value C_(F,MAX) satisfy the following inequality: C _(F,MAX)×1000>−8.97×1000×Δn ₃−61.54.
 4. The optical fiber according to claim 1, wherein, at said central core's outer radius r₁, said buried trench's refractive index difference Δn₃ and the atomic ratio of fluorine's maximum value C_(F,MAX) satisfy the following inequality: C _(F,MAX)×1000>−8.28×1000×Δn ₃−48.28.
 5. The optical fiber according to claim 1, wherein, at said central core's outer radius r₁, said buried trench's refractive index difference Δn₃ and the atomic ratio of fluorine's maximum value C_(F,MAX) satisfy the following inequality: C _(F,MAX)×1000>−8×1000×Δn ₃−42.28.
 6. The optical fiber according to claim 1, within said central core, the atomic ratio C_(F) of fluorine to all of the constituents of said doped core matrix is a function C_(F)(r) of the radial distance r from the center of the optical fiber and C_(F)(r) is defined by the following equation: ${C_{F}(r)} = {C_{F}^{(1)} + {\left( \frac{r}{r_{1}} \right)^{p} \cdot \left( {C_{F,{MAX}} - C_{F}^{(1)}} \right)}}$ where: p is a constant substantially equal to 2; and C_(F) ⁽¹⁾ is a positive constant less than 1.25×10⁻³.
 7. The optical fiber according to claim 1, wherein the alpha-index profile of the central core has an alpha parameter α of between about 1.9 and 2.1.
 8. The optical fiber according to claim 1, wherein the refractive index difference between said central core and said outer cladding has a maximum value Δn₁ of between about 10×10⁻³ and 18×10⁻³.
 9. The optical fiber according to claim 1, wherein the refractive index difference between said central core and said outer cladding has a maximum value Δn₁ of between about 11×10⁻³ and 16×10⁻³.
 10. The optical fiber according to claim 1, wherein: said inner cladding's width w₂ is between about 0.5 micron and 2 microns; and said inner cladding's refractive index difference Δn₂ is between about −0.2×10⁻³ and 2×10⁻³.
 11. The optical fiber according to claim 1, wherein said buried trench's width w₃ is between about 3 microns and 5 microns.
 12. The optical fiber according to claim 1, wherein the optical fiber has a numerical aperture of 0.2±0.015.
 13. The optical fiber according to claim 1, wherein, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of less than about 0.2 dB.
 14. The optical fiber according to claim 1, wherein, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of less than about 0.01 dB.
 15. The optical fiber according to claim 1, wherein, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 1300 nanometers, the optical fiber has bending losses of less than about 0.5 dB.
 16. The optical fiber according to claim 1, wherein, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 1300 nanometers, the optical fiber has bending losses of less than about 0.05 dB.
 17. The optical fiber according to claim 1, wherein: at a wavelength of 850 nanometers, the optical fiber has an overfilled launch (OFL) bandwidth of about 1500 MHz·km or greater; and/or at a wavelength of 1300 nanometers, the optical fiber has an overfilled launch (OFL) bandwidth of about 500 MHz·km or greater.
 18. The optical fiber according to claim 1, wherein: at a wavelength of 850 nanometers, the optical fiber has an overfilled launch (OFL) bandwidth of about 3500 MHz·km or greater; and/or at a wavelength of 1300 nanometers, the optical fiber has an overfilled launch (OFL) bandwidth of about 600 MHz·km or greater.
 19. The optical fiber according to claim 1, wherein, as acquired with a mask covering the outer portion of the central core in accordance with the FOTP-220 standard at a wavelength of 850 nanometers, the optical fiber has a dispersion mode delay DMDext value of about 0.33 ps/m or less.
 20. The optical fiber according to claim 1, wherein, as acquired with a mask covering the outer portion of the central core in accordance with the FOTP-220 standard at a wavelength of 850 nanometers, the optical fiber has a dispersion mode delay DMDext value of about 0.14 ps/m or less.
 21. A multimode optical system including the optical fiber according to claim 1, wherein the multimode optical system presents a data rate of about 10 Gb/s or greater over at least 100 meters.
 22. A multimode optical system including the optical fiber according to claim 1, wherein the multimode optical system presents a data rate of about 10 Gb/s or greater over at least 300 meters.
 23. An optical fiber, comprising: a central core surrounded by an outer optical cladding, said central core having an outer radius r₁ and an alpha-index profile with respect to said outer optical cladding, said central core comprising a core matrix doped with at least fluorine and a dopant element that increases refractive index; an inner cladding positioned between said central core and said outer cladding, said inner cladding having (i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive index difference Δn₂ with respect to said outer optical cladding; and a buried trench positioned between said inner cladding and said outer optical cladding, said buried trench having (i) an outer radius r₃, (ii) a width w₃, and (iii) a refractive index difference Δn₃ with respect to said outer optical cladding; wherein, within said central core, the atomic ratio C_(F) of fluorine to all of the constituents of said doped core matrix increases from said central core's center to said central core's outer radius r₁; wherein, at said central core's outer radius r₁, the atomic ratio C_(F) of fluorine to all of the constituents of said doped core matrix has a maximum value C_(F,MAX); and wherein, at said central core's outer radius r₁, said buried trench's refractive index difference Δn₃ and the atomic ratio of fluorine's maximum value C_(F,MAX) satisfy the following inequality: C _(F,MAX)×1000>−8.97×1000×Δn ₃−61.54.
 24. The optical fiber according to claim 23, wherein, as acquired with a mask covering the outer portion of the central core in accordance with the FOTP-220 standard at a wavelength of 850 nanometers, the optical fiber has a dispersion mode delay DMDext value of about 0.25 ps/m or less.
 25. An optical fiber, comprising: a central core surrounded by an outer optical cladding, said central core having an outer radius r₁ and an alpha-index profile with respect to said outer optical cladding, said central core comprising a core matrix doped with at least fluorine and a dopant element that increases refractive index; an inner cladding positioned between said central core and said outer cladding, said inner cladding having (i) an outer radius r₂, (ii) a width w₂, and (iii) a refractive index difference Δn₂ with respect to said outer optical cladding; and a buried trench positioned between said inner cladding and said outer optical cladding, said buried trench having (i) an outer radius r₃, (ii) a width w₃, and (iii) a refractive index difference Δn₃ with respect to said outer optical cladding; wherein, within said central core, the atomic ratio C_(F) of fluorine to all of the constituents of said doped core matrix increases from said central core's center to said central core's outer radius r₁; wherein, at said central core's outer radius r₁, the atomic ratio C_(F) of fluorine to all of the constituents of said doped core matrix has a maximum value C_(F,MAX); wherein, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of less than about 0.2 dB; wherein, for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 1300 nanometers, the optical fiber has bending losses of less than about 0.5 dB; wherein, at a wavelength of 850 nanometers, the optical fiber has an overfilled launch (OFL) bandwidth of about 1500 MHz·km or greater; and wherein, at a wavelength of 1300 nanometers, the optical fiber has an overfilled launch (OFL) bandwidth of about 500 MHz·km or greater.
 26. The optical fiber according to claim 25, wherein: for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 850 nanometers, the optical fiber has bending losses of less than about 0.1 dB; and for two turns around a radius of curvature of 7.5 millimeters at a wavelength of 1300 nanometers, the optical fiber has bending losses of less than about 0.2 dB. 